This invention relates to loop signaling and more particularly to method and apparatus for improving the return loss in a dial pulse termination channel unit (DPT) in a pulse code modulation (PCM) system.
A trunk circuit communication channel in a PCM system may comprise a dial pulse originating channel unit (DPO) and a DPT transmitting PCM signals therebetween. In establishing such a PCM communication channel, dialing of the originating or calling parties telephone causes the associated (far end) central office equipment to seize a DPO and transmit PF (pulse function) and CF (control function) signaling information in a PCM signal to a DPT at the near end. The DPT is responsive to a PF signal, indicating an off-hook condition of the calling parties handset, for drawing DC loop current from the near end central office battery on tip and ring lines in order to energize the A-relay there. The practice is to terminate the near end loop in the DPT with an 800 ohm resistor in series with a pair of secondary windings of the VF hybrid transformer there and contacts of a pulsing (PF) relay. Since the approximately 35 milliamperes of DC loop current that is required to energize the A-relay in the near end central office also flows through the secondary windings, it produces a DC saturation of the core of the VF transformer which reduces the transformer inductance and thus return loss at low frequencies. Return loss is defined here as the ratio of the VF power incident on the secondary windings from the near end central office to the power reflected from these windings. Additionally, a control (CF) relay is responsive to a CF signal for shunting these secondary windings and the 800 ohm resistor with a 200 ohm resistor for increasing the DC loop current and reducing distortion during dial pulsing. This structure is illustrated in FIG. 1. In order to maintain the inductance and return loss high, the VF transformer there is made physically large. This means that the transformer is large and expensive, and requires a large surface area on a printed circuit board. Since such a transformer is also relatively tall, this also means that PC boards cannot be located close together. An object of this invention is the provision of method and apparatus for diverting DC loop current away from windings of a transformer so that a smaller and more economical transformer, capable of providing higher inductance and return loss, may be used there.